US10253384B2 - Cold-rolled steel sheet, plated steel sheet, and method for manufacturing the same - Google Patents

Cold-rolled steel sheet, plated steel sheet, and method for manufacturing the same Download PDF

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US10253384B2
US10253384B2 US14/373,748 US201314373748A US10253384B2 US 10253384 B2 US10253384 B2 US 10253384B2 US 201314373748 A US201314373748 A US 201314373748A US 10253384 B2 US10253384 B2 US 10253384B2
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steel sheet
ferrite
annealing
under
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US20140352850A1 (en
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Yusuke Kondo
Kunio Hayashi
Toshio Ogawa
Naoki Matsutani
Koichi Goto
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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Assigned to NIPPON STEEL & SUMITOMO METAL CORPORATION reassignment NIPPON STEEL & SUMITOMO METAL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: GOTO, KOICHI, HAYASHI, KUNIO, KONDO, YUSUKE, MATSUTANI, NAOKI, OGAWA, TOSHIO
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
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    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
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    • C21D2211/00—Microstructure comprising significant phases
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    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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    • Y10T428/00—Stock material or miscellaneous articles
    • Y10T428/12—All metal or with adjacent metals
    • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
    • Y10T428/12771—Transition metal-base component
    • Y10T428/12785—Group IIB metal-base component
    • Y10T428/12792—Zn-base component
    • Y10T428/12799—Next to Fe-base component [e.g., galvanized]

Definitions

  • the present invention relates to a high-strength cold-rolled steel sheet and a plated steel sheet, which have excellent ductility and stretch flangeability and are suitable for an automobile steel sheet, particularly suitable for a structural member (for example, a bracket), and a method for manufacturing the same.
  • high-strengthening of steel to be used has rapidly progressed to improve fuel economy through a decrease in the weight of an automobile body and improve collision safety.
  • a high-strength steel sheet is called a “high tensile strength steel sheet”, and the amount of orders of thin steel sheets mainly having a tensile strength of 440 MPa to 590 MPa, and recently more than 590 MPa, tends to increase every year.
  • excellent ductility and stretch flangeability are demanded for a structural member such as a bracket in view of the working method.
  • a steel sheet having a uniform structure such as ferrite single phase steel or bainite single phase steel is considered to be superior. From the above viewpoint, it is important to control a constituent phase fraction matched with a desired tensile strength to attain both ductility and hole expansibility.
  • the maximum heating temperature in the annealing step becomes extremely high.
  • the maximum heating temperature is just higher than an Ac 1 transformation temperature, a temperature rising rate becomes extremely slow.
  • Patent Document 1 Japanese Unexamined Patent Application, First Publication No. 2010-285656
  • Patent Document 2 Japanese Unexamined Patent Application, First Publication No. 2010-285657
  • Patent Document 3 Japanese Unexamined Patent Application, First Publication No. 2008-106352
  • Patent Document 4 Japanese Unexamined Patent Application, First Publication No. 2.008-190032
  • Patent Document 5 Japanese Unexamined Patent Application, First Publication No. 2009-114523
  • the present invention is to stably provide a high-strength cold-rolled steel sheet and a plated steel sheet which have excellent ductility and stretch flangeability, without deterioration in productivity.
  • the present invention is a finding obtained from an investigation that has been conducted to solve the above mentioned problems of improving ductility and stretch flangeability of a high-strength cold-rolled steel sheet, a hot-dip galvanized steel sheet, and a galvannealed steel sheet which have a tensile strength of 590 MPa or more. That is, an appropriate microstructure is attained by optimizing the amount of alloying elements, particularly, optimizing the amount of Nb, and Ti and positively adding Si. In addition, in an annealing process, the maximum heating temperature is controlled within a temperature range of Ac 1 [° C.] or more and (Ac 1 +40) [° C.] or less and an end temperature and a cooling rate of a primary cooling after annealing are determined.
  • the present invention is made based on the findings that it is possible to produce a steel sheet having excellent ductility and stretch flangeability compared to steel sheets of the related art, and the summary thereof is described as follows.
  • a cold-rolled steel sheet including, by mass %: C: 0.020% or more and 0.080% or less; Si: 0.20% or more and 1.00% or less; Mn: 0.80% or more and 2.30% or less; P: 0.0050% or more and 0.1500% or less; S: 0.0020% or more and 0.0150% or less; Al: 0.010% or more and 0.100% or less; N: 0.0010% or more and 0.0100% or less; and further including: one or more of Nb and Ti which satisfy a requirement of 0.005% ⁇ Nb+Ti ⁇ 0.030%; and a reminder including Fe and unavoidable impurities, in which a structure consists of, a ferrite, a bainite, and an other phase, the other phase includes a pearlite, a residual austenite, and a martensite, an area ratio of the ferrite is 80% or more and less than 95%, an area ratio of a non
  • the cold-rolled steel sheet according to (1) may further include one or more of, by mass %; Mo: 0.005% or more and 1.000% or less; W: 0.005% or more and 1.000% or less; V: 0.005% or more and 1.000% or less; B: 0.0005% or more and 0.0100% or less; Ni: 0.05% or more and 1.50% or less; Cu: 0.05% or more and 1.50% or less; and Cr: 0.05% or more and 1.50% or less.
  • a plated steel sheet in which plating is provided on a surface of the cold-rolled steel sheet according to (1) or (2).
  • a method for manufacturing a cold-rolled steel sheet including: heating a slab having a chemical composition according to (1) or (2) to 1150° C. or more and 1280° C. or less; finishing a finish rolling under a temperature of Ar 3 ° C. or more and 1050° C. or less; pickling and then cold-rolling a hot-rolled steel sheet, which is coiled under a temperature range of 450° C. or more and 650° C. or less, under a reduction of 40% or more and 70% or less; thereafter heating into a temperature range of Ac 1 ° C. or more and (Ac 1 +40)° C.
  • Ar 3 910 ⁇ 325 ⁇ [C]+33 ⁇ [Si]+287 ⁇ [P]+40 ⁇ [Al] ⁇ 92 ⁇ ([Mn]+[Mo]+[Cu]) ⁇ 46 ⁇ ([Cr]+[Ni]) (Expression 1).
  • Ac 1 723+212 ⁇ [C] ⁇ 10.7 ⁇ [Mn]+29.1 ⁇ [Si] (Expression 2), and
  • a method for manufacturing a plated steel sheet including plating the cold-rolled steel sheet manufactured by the method according to (4) after the annealing and the cooling.
  • the method for manufacturing a plated steel sheet according to (5) may further include heat treating the plated steel sheet under a temperature range of 450° C. or more and 600° C. or less with 10 seconds or longer.
  • the present invention it is possible to provide a high-strength cold-rolled steel sheet, a hot-dip galvanized steel sheet, and a galvannealed steel sheet which have a tensile strength of 590 MPa or more, and excellent ductility and stretch flangeability. Therefore, the present invention makes an extremely significant contribution to the industry.
  • FIG. 1 is a graph showing a relationship between the maximum heating temperature, particularly, an Ac 1 transformation point or higher, in annealing and an area ratio of non-recrystallization ferrite.
  • FIG. 2 is a graph showing a relationship between an area ratio of non-recrystallization ferrite and a hole expansion ratio ⁇ .
  • FIG. 3 is a graph showing a relationship between the maximum heating temperature, particularly, an Ac 1 transformation point or higher, in annealing and an area ratio of bainite.
  • FIG. 4 is a graph showing a relationship between an area ratio of bainite and a hole expansion ratio ⁇ .
  • FIG. 5 is a graph showing a relationship between the maximum heating temperature, particularly, an Ac 1 transformation point or higher, in annealing and an equivalent circle diameter of carbonitrides.
  • FIG. 6 is a graph showing a relationship between an equivalent circle diameter of carbonitrides and an area ratio of non-recrystallization ferrite.
  • FIG. 7 is a graph showing a relationship between a total amount of other phase and a hole expansion ratio ⁇ .
  • C is an element which contributes to an increase in tensile strength and yield strength, and added in appropriated amounts according to a targeted strength level. In addition, C is also effective in obtaining bainite.
  • the lower limit is set to 0.020%.
  • the upper limit is set to 0.080%.
  • the lower limit of C may be preferably 0.030% or 0.040%, and the upper limit of C may he preferably 0.070% or 0.060%.
  • Si is an extremely important element in the present invention. Si is effective in improving the stretch flangeability by hardening ferrite through solid solution strengthening to reduce the hardness difference with a hard phase. In order to obtain the effect, it is necessary to set the amount of Si to 0.20% or more, and thus, the lower limit is set to 0.20%. On the other hand, Si may cause a problem of a decrease in plating wettability when hot dip galvanizing is carried out and a problem of a decrease in productivity due to the delay of alloying reaction. Therefore, the upper limit of the amount of Si is set to 1.00%. In addition, Si is a ferrite stabilizing element. The lower limit of Si may be set to 0.30% or 0.40% and the upper limit of Si may be set to 0.90% or 0.80% to obtain an appropriate amount of bainite.
  • Mn acts as an element that contributes to solid solution strengthening, and thus has an effect of increasing the strength.
  • Mn is effective in obtaining beinite.
  • it is necessary to contain 0.80% or more of Mn to improve hole expansibility.
  • the upper limit thereof is set to 2.30%.
  • the lower limit of Mn may be set to 1.00%, 1.20%, or 1.80%, and the upper limit of Mn may be set to 2.10%, or 2.00%.
  • the upper limit of the amount of P is set to 0.1500%. Since P is an element which increases strength at a low price, the lower limit of the amount of P is preferably set to 0.0050% or more. In order to further improve the toughness and the weldability, the lower limit of P may be set to 0.0060% or 0.0070%, and the upper limit of P may be set to 0.1000% or 0.0850%.
  • the upper limit of the amount of S is set to 0.0150%. Due to restriction on production costs, the lowver limit of the amount of S is set to 0.0020%. In order to improve the workability, the lower limit of S may be set to 0.0025%, and the upper limit of S may be set to 0.0100% or 0.0080%.
  • Al is a ferrite stabilizing element similar to Si.
  • Al is a deoxidizing element and the lower limit is set to 0.010% or more in view of deoxidation.
  • the upper limit thereof is set to 0.100%.
  • the lower limit of Al may be set to 0.015% or 0.025%, and the upper limit of Al may be set to 0.080%, 0.060% or 0.040%.
  • N is an impurity. When the amount of N is more than 0.0100%, deterioration in toughness and ductility and occurrence of cracking in a slab are significant. Since N is effective in increasing tensile strength and yield strength, similar to C, N may be positively added as the upper limit of the amount of N is set to 0.0100%. In addition, N is effective in obtaining bainite. Due to restriction on production costs, the lower limit of the amount of N is set to 0.0010%. The lower limit of N may be set to 0.0020% or 0.0030%, and the upper limit of N may be 0.0080%, 0.0060%, or 0.0050%.
  • Nb and Ti are extremely important elements in the present invention. These elements have an effect of delaying the progress of recrystallization in an annealing process to allow non-recrystallization ferrite to remain. Since the non-recrystallization ferrite contributes to hardening of ferrite, the amount of the non-recrystallization ferrite is appropriately controlled to reducing the hardness difference between ferrite and the hard phase, thereby obtaining an effect of improving the stretch flangeability.
  • the reason why the upper limit of at least one of Nb and Ti is less than 0.030% is that when one or more of Nb and Ti are added at a content more than the upper limit, the non-recrystallization ferrite remains excessively, and the ductility is decreased.
  • the reason why the lower limit of one or more of Nb and Ti are set to 0.005% is that when one or more of Nb and Ti are added at a content less than the lower limit, a recrystallization suppressing effect is small, and the non-recrystallization ferrite hardly remains.
  • the lower limit of one or more of Nb and Ti may be set to 0.010%, and the upper limit of one or more of Nb and Ti may be set to 0.025%.
  • All of Mo, W, and V are recrystallization inhibiting elements, and one or more of these elements may be added as required.
  • 0.005% of Mo, 0.005% of W, and 0.005% of V are preferably added respectively as the lower limits.
  • the upper limits are preferably set to 1.000% of Mo, 1.000% of W, and 1.000% of V, respectively.
  • All of B, Ni, Cu, and Cr are elements which increase hardenability, and one or more of these elements may be added as required.
  • 0.0005% of B, 0.05% of Ni, 0.05% of Cu, and 0.05% of Cr are preferably added respectively as the lower limits.
  • the upper limits are preferably set to 0.0100% of B, 1.50% of Ni, 1.50% of Cu, and 1.50% of Cr, respectively.
  • the high-strength cold-rolled steel sheet containing the above-described chemical composition may contain impurities unavoidably incorporated in a production process within the range in which a reminder including iron as a main component does not impair the properties of the present invention.
  • a slab having the above-described composition is heated at a temperature of 1150° C. or higher.
  • the slab may be a slab immediately after being produced by a continuous casting facility or a slab produced by an electric furnace.
  • the reason why the temperature is limited to 1150° C. or higher is to sufficiently decompose and dissolve carbonitride forming elements and carbon in the steel.
  • the temperature is preferably 1200° C. or higher.
  • the heating temperature is higher than 1280° C., the temperature is not preferable in view of production costs, and thus, 1280° C. is preferably set as the upper limit.
  • a finishing temperature in hot rolling is lower than an Ar 3 transformation temperature, carbonitrides are precipitated and the particle size is coarsened in the surface area, and the tensile strength and stretch flangeability are decreased after the annealing, so that the Ar 3 transformation temperature is set as the lower limit.
  • a temperature of 900° C. or higher is preferable to stably precipitate the precipitates of carbonitrides with an equivalent circle diameter of 10 nm or less.
  • the upper limit of the finishing temperature is substantially 1050° C. in view of the slab heating temperature.
  • Ar 3 ° C. is an Ar 3 transformation temperature obtained by the following Expression 1.
  • Ar 3 910 ⁇ 325 ⁇ [C]+33 ⁇ [Si]+287 ⁇ [P]+40 ⁇ [Al] ⁇ 92 ⁇ ([Mn]+[Mo]+[Cu]) ⁇ 46 ⁇ ([Cr]+[Ni]) (Expression 1)
  • a coiling temperature after finishing rolling is an extremely important production condition in the present invention.
  • the suppression of the precipitation of carbonitrides at the stage of the hot-rolled steel sheet with setting the coiling temperature to 650° C. or lower is important, and the properties of the present is not deteriorated by the history up to that time.
  • the coiling temperature is higher than 650° C.
  • carbonitrides are precipitated and coarsened in the hot-rolled steel sheet, sufficient crystallization suppressing effects cannot be attained during annealing, and thus, 650° C. is set as the upper limit.
  • the coiling temperature is lower than 450° C., the strength of the hot-rolled steel sheet is increased and rolling load is increased during cold rolling. Therefore, 450° C. is set as the lower limit.
  • the reduction is less than 40%, the driving force of recrystallization becomes small during the annealing, and thus, non-recrystallization ferrite remains excessively after the annealing, which causes a decrease in the ductility.
  • the lower limit is set to 40%.
  • the driving force of recrystallization becomes large during the annealing, and thus, a small amount of non-recrystallization ferrite remains, which causes a decrease in the tensile strength and the stretch flangeability. Therefore, the upper limit is set to 70%.
  • the annealing is preferably carried out by the continuous annealing facility to control the heating temperature and the heating time.
  • the maximum heating temperature in the annealing is an extremely important production condition in the present invention.
  • the lower limit of the maximum heating temperature is set to an Ac 1 transformation temperature, and the upper limit is set to (Ac 1 transformation temperature+40)° C.
  • the maximum heating temperature is lower than the Ac 1 transformation temperature, a sufficient amount of a hard phase and non-recrystallization ferrite are not obtained, and a decrease in the tensile strength is caused.
  • the maximum heating temperature is higher than (Ac 1 transformation temperature+40)° C., the amount of the non-recrystallization ferrite is reduced as shown in FIG.
  • Ac 1 ° C. is an Ac 1 transformation temperature obtained by the following Expression 2.
  • Ac 1 723+212 ⁇ [C] ⁇ 10.7 ⁇ [Mn]+29.1 ⁇ [Si] (Expression 2)
  • a temperature rising rate is set to 2° C./sec to 5° C./sec in the annealing.
  • the temperature rising rate is less than 2° C./sec, not only is the productivity deteriorated, but also recrystallization substantially proceeds to reduce the amount of non-recrystallization ferrite, and thus, the tensile strength and the stretch flangeability are decreased. Therefore, the lower limit is set to 2° C./sec.
  • the upper limit is set to 5° C./sec.
  • a holding time at the maximum heating temperature in the annealing is an extremely important production condition in the present invention.
  • the holding time of the steel sheet within the temperature range of the Ac 1 transformation temperature to (Ac 1 transformation temperature+40)° C. is set to 10 seconds to 200 seconds. This is because when the holding time of the steel sheet at the maximum heating temperature is shorter than 10 seconds, non-recrystallization ferrite remains excessively, and thus, the ductility is decreased.
  • the holding time of the steel sheet at the maximum heating temperature is increased, a decreased in the productivity is caused and also the amount of non-recrystallization ferrite is reduced. Then, the tensile strength and the stretch flangeability are decreased, and thus, the upper limit is set to 200 seconds.
  • primary cooling for cooling the steel within a steel sheet temperature range of 600° C. to 720° C. tray be carried out under a cooling rate of 10° C./sec or less.
  • the steel sheet may be cooled and controlled to an appropriate temperature through forced cooling with spraying of a coolant, such as water, air blowing, or mist or the like, and over-aging or tempering is additionally carried out during the cooling as required.
  • a coolant such as water, air blowing, or mist or the like
  • the cooling rate is more than 10° C./sec, the precipitation of ferrite is small and the fraction of bainite becomes excessive, and thus, the ductility is decreased.
  • the lower limit of the cooling rate is not particularly limited, but is preferably set to 1° C./sec or more in view of the productivity and the cooling controllability.
  • the composition of zinc plating is not particularly limited, and in addition to Zn, Fe, Al, Mn, Cr, Mg, Pb, Sn, Ni, and the like may be added as required.
  • the plating may be carried out as a separate process from annealing, but is preferably carried out through a continuous annealing-hot dip galvanizing line in which annealing, cooling and plating are continuously carried out in view of the productivity.
  • the steel sheet is cooled to a normal temperature after the plating.
  • the alloying treatment be carried out within a temperature range of 450° C. to 600° C. after the plating, and then, the steel sheet be cooled to a normal temperature. This is because alloying does not sufficiently proceed at a temperature of lower than 450° C., and alloying excessively proceeds at a temperature of higher than 600° C. such that the plated layer is embrittled to cause a problem of exfoliation of the plating by working such as pressing or the like.
  • an alloying treatment time is shorter than 10 seconds, alloying does not sufficiently proceed, and thus, 10 seconds or longer is preferable.
  • the upper limit of the alloying treatment time is not particularly limited, but preferably within 100 seconds in view of productivity.
  • an alloying treatment furnace be provided continuously to the continuous annealing-hot dip galvanizing line to carry out annealing cooling, plating and an alloying treatment, and cooling in a continuous manner.
  • Examples of the plated layer shown in examples include hot dip galvanizing and galvannealing, and electrogalvanizing is also included.
  • Skin pass rolling is carried out to correct the shape and secure the surface properties, and is preferably carried out in a range of an elongation ratio of 0.2% to 2.0%.
  • the reason why the lower limit of the elongation ratio of the skin pass rolling is set to 0.2% is that sufficient improvement in the surface roughness is not attained at an elongation ratio of less than 0.2%, and thus, the lower limit is set to 0.2%.
  • the upper limit is set to 2.0%.
  • the microstructure of the steel sheet obtained by the present invention is composed of mainly ferrite and bainite.
  • the area ratio of ferrite is less than 80%, bainite is increased and sufficient ductility cannot be obtained.
  • the lower limit of the area ratio of ferrite is set to 80%.
  • the area ratio of ferrite is 95% or more, a tensile strength of 590 MPa or more cannot be secured in some cases, and thus, the upper limit of the area ratio of ferrite is set to less than 95%.
  • the area ratio of ferrite is preferably 90% or less.
  • the non-recrystallization ferrite contributes to hardening of the ferrite, the effect of improving the stretch flangeability is obtained by reducing the hardness difference with the bainite with appropriately controlling the area ratio of the non-recrystallization ferrite within a range of 1% or more and less than 10%.
  • the ratio of the non-recrystallization ferrite in the ferrite is less than 1%, the non-recrystallization ferrite does not contribute to hardening of the ferrite, and thus, the lower limit of the area ratio of the non-recrystallization ferrite is set to 1% or more.
  • the ratio of the non-recrystallization ferrite in the ferrite is 10% or more, a decrease in the hole expansion ratio or the like is caused, and thus, the upper limit is set to less than 10%.
  • Bainite contributes to high-strengthening. However, when the amount of bainite is excessive, a decrease in the ductility is caused, and thus, the lower limit is set to 5% and the upper limit is set to 20%.
  • the upper limit of the total amount of the pearlite, residual austenite, and martensite is set to less than 8%.
  • a tensile strength of 590 MPa or more can be obtained.
  • the upper limit of the tensile strength is not particularly limited. However, considering the lower limit of the area ratio of the ferrite of the present invention, the upper limit may be set to about 780 MPa.
  • the equivalent circle diameter of the carbonitrides including one or both of Nb and Ti is set to 10 nm or less.
  • the average particle diameter of the carbonitrides is extremely important to appropriately control the amount of the non-recrystallization ferrite, and when the equivalent circle diameter is more than 10 nm, a sufficient recrystallization suppressing effect cannot be obtained and an appropriate amount of the non-recrystallization ferrite cannot be obtained.
  • the upper limit is set to 10 nm.
  • the lower limit is set to 1 nm or more in terms of accuracy in measurement.
  • the microstructure may be observed with an optical microscope by collecting a sample having an observation surface which is a cross section parallel to the rolling direction and the thickness direction, polishing the observation surface, and carrying out nital etching, and as required, Le Pera etching.
  • an optical microscope In the observation of the microstructure, regarding the sample collected from an arbitrary position of the steel sheet, a portion which is at a 1 ⁇ 4 portion along the thickness direction was imaged at a magnification of 1000 times in a range of 300 ⁇ 300 ⁇ m.
  • the image of the microstructure obtained by the optical microscope is analyzed by binarizing the image to white and black so that a total area ratio of any one or two or more of pearlite, bainite, and martensite can be obtained as an area ratio of phases other than the ferrite.
  • the fraction was measured for the sample collected from arbitrary position of the steel sheet with by the above method imaging a 1 ⁇ 4 portion along the thickness direction at a magnification of 1000 times in a range of 300 ⁇ 300 ⁇ m and having 3 or more imaged view fields. It is difficult to distinguish residual austenite from martensite with the optical microscope, but the volume ratio of the residual austenite can be measured by an X-ray diffraction method. The sample used in the aforementioned microstructure observation is used for obtaining the fraction of the residual austenite.
  • the non-recrystallization ferrite and ferrite other than the non-recrystallization ferrite can be determined by analyzing the measurement data of the orientation of an electron back scattering pattern (EBSP) by the Kernel Average Misorientation method (KAM method).
  • EBSP electron back scattering pattern
  • KAM method Kernel Average Misorientation method
  • the area ratio of the non-recrystallization ferrite is an area ratio obtained by subtracting the area ratio of ferrite other than the non-recrystallization ferrite from the area ratio of total ferrite.
  • the area ratio obtained from the microstructure is the same as the volume ratio.
  • the average particle size of the carbonitrides including one or both of Nb and Ti is measured by preparing an extraction replica sample extracted from a portion which is at a depth of 1 ⁇ 4 of the sheet thickness from a surface of arbitrary position of the steel sheet, and observing carbonitrides as a target with a transmission type electron microscope (TEM) to obtain the average particle size of the carbonitrides.
  • the average particle size was obtained by imaging an image at a magnification of 10000 times in a range of 10 ⁇ 10 ⁇ m, and counting 100 random particles of alloy carbides. It is difficult to count a particle having a size of 1 nm, and 100 large particles are counted, not exactly in descending order, but in random order.
  • a test method of each mechanical property will be described below.
  • a tensile test sample according to JIS Z 2201 No. 5 was taken from a steel sheet after being manufactured in which the width direction (referred to as the TD direction) is considered as the longitudinal direction, and the tensile properties in the TD direction were evaluated according to JIS Z 2241.
  • the stretch flangeability was evaluated according to Japan Iron and Steel Federation Standard JFS T 1001.
  • Each of the obtained steel sheets was cut to 100 mm ⁇ 100 mm size pieces and then punched to have a hole with a diameter of 10 mm with a clearance being 12% of the thickness.
  • D f represents a hole diameter [mm] at the time of crack initiation
  • D 0 represents an initial hole diameter [mm].
  • plating adhesion is evaluated according to JIS H 0401 by visually observing a surface state of a plating film at a portion bent by a bending test.
  • the stretch flangeability was evaluated according to Japan Iron and Steel Federation Standard JFS T 1001.
  • Each of the obtained steel sheets was cut to 100 mm ⁇ 100 mm size pieces and then punched to have a hole with a diameter of 10 mm with a clearance being 12% of the thickness.
  • D f represents a hole diameter [mm] at the time of crack initiation
  • D 0 represents an initial hole diameter.
  • plating adhesion is evaluated according to JIS H 0401 by visually observing a surface state of a plating film at a portion bent by a bending test.
  • the microstructure of the sheet thickness cross section of the steel sheet was observed by the above-described manner, and the area ratio of bainite was obtained as a total area ratio of phase which is not ferrite and other phases.
  • the holding time at the maximum heating temperature during the annealing is short, the amount of bainite is reduced and the area ratio of non-recrystallization ferrite is increased.
  • the total elongation is decreased, the product of tensile strength and total elongation is decreased and the hole expansion ratio is also decreased.
  • the present invention it is possible to provide a high-strength cold-rolled steel sheet and a plated steel sheet which have a tensile strength of 590 MPa or more, and excellent ductility and stretch flangeability, and the present invention makes an extremely significant contribution to the industry.

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